Ok, break down and add some cast<>'ing helper methods to the Type class

Ok, break down and add some cast<>'ing helper methods to the Type class
to reduce the number of cast<>'s we have.  This allows someone to use
things like Ty->getVectorNumElements() instead of
cast<VectorType>(Ty)->getNumElements() when you know that a type is a
vector.

It would be a great general cleanup to move the codebase to use these,
I will do so in the code I'm touching.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148999 91177308-0d34-0410-b5e6-96231b3b80d8